serial number suggests a single barrel, hinged frame Model 1908/08 shotgun - if so - 1920-25.
M prefix to serial # = 1952 manufacture. Jim
"AD" prefix indicates 1967
That should be 1975
"AX" prefix indicates 1981
The prefix letter shows the year of manufacture. 1956 is letter S
Harrington & Richardson used the Prefix AT in 1983.
How old is my Topper 88 HARRINGTON AND RICHARDSON GUN serial number
"P" prefix indicates 1953
Harrington and Richardson used serial numbers that started with AM in 1974. The company had similar shotguns being built from 1974 to 1981.
what letter or letters are in front of serial number